"People react to these things when they see thresholds crossed," David Etheridge, a CSIRO principal research scientist, told us. Even more upbeat was Paul Fraser, the CSIRO scientist who had helped set up Cape Grim 40 years ago. He told 9news.com.au that passing the landmark would be a "psychological tipping point".

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Smoke rises from Canadian wildfires burning near Fort McMurray, Alberta as the fire season started a month early this year.Credit:Darryl Dyck, Bloombeg

Other warning signs - such as the 1 degree warming point reached last year - did at least spur nations to agree at the Paris Climate Summit that temperature increases should be limited to 1.5-2 degrees, even if national policies made 3 degrees more likely.

And so, with CO2 concentrations marching past 400 ppm, when should we start worrying about global warming?

"About 30 years ago," is the blunt answer from David Karoly, a climate scientist with Melbourne University. That's when CSIRO and other scientists declared we had a problem.

"But we shouldn't give up, either," he told me this week. "The worry - and the [climate] action - should now be increased."

No sign of a cooling off in global temperatures.Credit:Peter Rae

During the three decades since, we have poured trillions of dollars into fossil fuel extraction, refining and consumption and a much smaller sum into renewable energy that will have to replace coal, oil and gas - and soon.

(See NASA's chart showing temperatures were 1.11 degrees warmer than the 1951-80 average, with large areas in the north particularly warm, while Australia had its second-warmest April.)

Among the climate-related threats, Canada's forest fire season started about a month early. One huge blaze near Alberta's tar sands forced the evacuation of about 88,000 people from the city of Fort McMurray.

The monster El Nino, riding on top of about one degree of background warming, has triggered widespread bleaching of the corals around the world, including our cherished - but apparently expendable - Great Barrier Reef.

On the plus side

And all that extra CO2 is leading to a "greening" of the planet with about one-quarter of humans' carbon emissions being taken up by plants, a recent Nature paper found.

But many other changes are far from benign - at least according to the preservation of many existing eco-systems.

Less glamorous than corals, the kelp forests off eastern Tasmania are being destroyed by warm water species swept south by the strengthening and warming East Australian Current. Similarly, the massive dieback of mangrove forest in the Gulf of Carpentaria apparently linked to extreme weather got scant national media coverage.

And, as for the high Arctic where the remarkably prolonged above-average temperatures have led to record-low levels of sea ice, changes are likely to be further from our daily minds.

But less sea ice means less light reflected back to space and more heat absorbed by the oceans - and so the cycle builds and so should our concern.

Spiral warning

In case you missed it, Ed Hawkins, a climate scientist at the UK's University of Reading, came up with the illustration below to help us appreciate the spiralling warming trend since the middle of the 19th century.

By that gauge, when the temperature increase breaks past the 1.5 degrees warming - perhaps in coming months before the El Nino in the Pacific fully stops giving back heat to the atmosphere - will be sufficiently concerned then to act?

Climate scientists, such as Professor Karoly, know globally warming limits agreed at international summits to prevent dangerous climate change - such as 1.5-2 degrees range agreed in Paris - are arbitrary in a similar way to 400 ppm.

"You'd be hard-pressed to find scientists agreeing that 1.5 degrees would be manageable for our reef," he said. "Look at what's happening to the reef this year - and that's with 1 degree warming."

Recent work by his team has identified that on current temperature and carbon emissions trends, the Great Barrier Reef will be hit by bleaching events every second year by 2035.

Other eco-systems, such as alpine ones, are also being affected as species shielded from predators by cool temperatures have literally nowhere to hide.

Big one to worry about

Natural fluctuations mean temperature spikes such as we have seen over the past year can recede - at least partly.

As this year's El Nino in the Pacific lurches towards becoming a La Nina - when equatorial winds turn back to be mostly west-ward blowing and strengthen - we can expect the run of record temperature reading to be broken.

However with greenhouse gas concentrations still rising, the heat we are trapping in the Earth's biosphere - the land, air and sea - will keep on increasing.

And for those climate sceptics who thought the first decade of the 21st century marked a slowdown in temperature increases - the so-called "warming hiatus" - their case for holding off carbon emission cuts is about to get harder to make.